Experimental Study of Asphaltene Precipitation and Deposition During Immiscible CO2 - EOR Process

The Enhanced Oil Recovery (EOR) assisted with CO2 injections has been widely addressed. However, limited experimental work has been done for studying the asphaltene precipitation arising during the immiscible CO2 injection processes. This investigation presents experimental phase behavior analyses of asphaltenes instability, determination of asphaltene onset pressures (AOP), characterization, and description of asphaltene precipitation and deposition particles under different reservoir conditions. An advanced fully visual Pressure-Volume-Temperature (PVT) instrument and a Solid Detection System (SDS) are utilized to perform all the measurements in this work. Saturation pressures are measured for the gas and crude oil mixtures with different compositions under different reservoir temperatures in the PVT cell. Changes in pressure, temperature, and volume at each equilibrium state are recorded. The same mixture composition is charged into the SDS. The AOP is then determined by reducing the pressure in the SDS. The upper asphaltene onset pressure (UAOP) is found to increase with the CO2 concentration in the system from 25 to 35 mol% CO2. For 45 mol% CO2, the UAOP is found to be lower than UAOP at 25 mol% CO2. In contrast, the lower asphaltene onset pressure (LAOP) is found to increase with CO2 concentration in all cases. The reversible process of asphaltene precipitation during the de-pressurization process at constant temperature is corroborated with the experiment at 60, 90, and 120 °C for the composition of 25 mol% of CO2 and at 90 and 120 °C for the composition of 35 mol% of CO2. The rest of the cases presented asphaltene deposition, which is considered an irreversible process because the asphaltene particles cannot be re-peptized into the liquid phase by the effect of pressure. By contrast, the complete re-peptization of asphaltenes during the re-pressurization is more effective at lower gas injection fractions (25, 35 mol% CO2) and higher temperatures (90, 120 °C). The maximum quantity and size of asphaltene particles are found near the bubble point pressure for all cases. The asphaltenes particles do not have a specific shape, and their colors vary from brown to black. The amount of asphaltene precipitation increase with the gas fraction by the effect of the micro-aggregates-clusters formation being able to reach the stage of aging or irreversible asphaltene deposition. Conversely, lower asphaltene precipitation is shown with the increment of temperature for 25 mol% CO2. This experimental work attempts to analyze the asphaltene precipitation phase behavior and particles observation relationship when CO2 is injected at different gas proportions and under different reservoir temperatures. The results from this effort provide significant support to the areas of asphaltene phase behavior characterization and formation damage control.